This invention relates to a process for oxidizing H.sub.2 S, and particularly to a process for catalytically oxidizing H.sub.2 S to sulfur, SO.sub.2, or both in the presence of a substantial proportion of water vapor.
Current air pollution regulations are very restrictive concerning the amount of H.sub.2 S that may be discharged to the atmosphere. In some instances, gas streams may not be discharged to the atmosphere if they contain more than about 10 ppmv of H.sub.2 S. Thus, many processes have been developed to remove H.sub.2 S from gas streams prior to their discharge to the atmosphere.
One method known in the art for removing H.sub.2 S involves catalytic oxidation, that is, a gas stream containing H.sub.2 S is blended with air or free oxygen, and the resulting mixture is then passed through a bed of catalyst particles under appropriate conditions such that the H.sub.2 S is converted to elemental sulfur vapor or SO.sub.2, or both, as desired. One catalyst useful for the gas phase conversion of H.sub.2 S to sulfur or SO.sub.2 is disclosed in U.S. Pat. No. 4,092,404; it comprises one or more vanadium oxides or sulfides supported on a refractory oxide such as alumina or silica-alumina. Another such catalyst is disclosed in U.S. Pat. No. 4,012,486, wherein a catalyst having active components consisting of bismuth is used to catalytically incinerate H.sub.2 S to SO.sub.2.
When compared, the bismuth catalyst of U.S. Pat. No. 4,012,486 will generally be found to be less active than the vanadia catalyst of U.S. Pat. No. 4,092,404 for oxidizing H.sub.2 S to SO.sub.2. On the other hand, a bismuth catalyst is much more stable than a vanadia catalyst at operating temperatures below about 600.degree. F. when H.sub.2 S must be removed from a gas stream, such as an off-gas derived from a geothermal power plant, which contains water vapor at a water vapor partial pressure above 1.0 psia, usually at least 4.0 psia. In general, vanadia catalysts have satisfactory stability in the presence of water vapor at partial pressures below about 1.0 psia or at operating temperatures above about 600.degree. F., but under the combined conditions of temperature below 600.degree. F. and water vapor partial pressures above about 1.0 psia, and particularly at 1.5 psia or above, vanadia catalysts deactivate rapidly. It is believed that the reason for this deactivation is due to a complex series of chemical reactions involving the conversion of the vanadium oxide or sulfide active catalytic components to less active forms of vanadium, such as vanadyl sulfate (VOSO.sub.4).
As stated above, vanadia catalysts are highly active for the oxidation of H.sub.2 S and, as disclosed in U.S. Patent application Ser. No. 700,513, filed June 28, 1976, now U.S. Pat. No. 4,243,647, such catalysts have proven most useful for oxidizing H.sub.2 S to sulfur by reaction with either oxygen or SO.sub.2. In the presence of less than about 1.0 psia water vapor, vanadia catalysts have proven to be remarkably stable, providing high conversions of H.sub.2 S to sulfur for a time period of more than 1 year with little if any deactivation being noticed. Despite the remarkable properties of vanadia catalysts, however, it is an object of the present invention not only to improve the stability of vanadium-containing catalysts in the presence of water vapor but to substantially improve their activity for converting H.sub.2 S to sulfur. More specifically with respect to water vapor, it is an object of the invention to provide a process for catalytically oxidizing H.sub.2 S in the presence of water vapor at a partial pressure of more than about 1.0 psia, particularly above about 1.5 psia, and more particularly still, above about 2.0 psia. It is yet another object of the invention to achieve the foregoing without oxidizing such component as H.sub.2, CO, NH.sub.3, and CH.sub.4 that might be present during the oxidation of the H.sub.2 S. Other objects and advantages will become apparent from the following description of the invention.